1. Field of the Invention
The present invention relates to a centrifuge, and more particularly to a centrifuge used for a sample separation.
2. Description of Related Art
In a conventional centrifuge, torque is produced by a power generator (generally an electric motor) that is part of a drive unit, and transferred to a rotor via a rotating shaft, causing the rotor to rotate. There are several types of rotors used for this type of centrifuge according to the type and amount of sample being separated and the like. These rotor types include a fixed-angle rotor provided with a plurality of holes at fixed angles for holding sample tubes that have been injected with samples, a swing-bucket rotor having vessels called buckets that are swingably supported on rotor arms so as to be able to swing independently while the rotor arms rotate and capable of holding a plurality of sample tubes, and a horizontal rotor in which sample tubes are mounted in a horizontal state.
In general, a rotor suitable for the intended purpose is selected from one of the above rotors. Vessels, such as sample tubes that have been injected with a sample, are inserted into the rotor, and the rotor is rotated, generating a centrifugal force for separating the sample or for shaking off droplets or the like deposited on the side walls of the sample tubes. However, since the user is responsible for injecting samples in the tubes and inserting sample tubes into the rotor, the manufacturer cannot guarantee a precise balance in the rotor.
For example, blood tests, which are widely used for medical diagnoses and the like, generally employ vacuum blood collection tube for drawing blood from the patient. However, the amount of blood that is drawn depends upon the patient and the person drawing the blood, and the evacuated tubes mounted in the rotor vary in number and weight. As a result, the centrifuge commonly operates in an imbalanced state, even when the user takes balance into consideration. Accordingly, manufacturers have been committed to developing sturdy devices that can withstand imbalances as much as possible and design devices that can allow imbalances to a certain degree.
If the imbalance exceeds a predetermined amount, then the force of the imbalance that increases as the rotations increase has an adverse effect on the bearing supporting the rotating shaft, which can bend the rotating shaft or cause other problems. Further, it is inevitable that imbalance exceeds the allowable amount when the user mistakenly injects the wrong amount of a sample or inserts the sample tubes in the wrong sample tube holes. Accordingly, most centrifuges are provided with sensors for detecting vibrations or amplitude. When the sensors detect that the centrifuge is operating in a state of imbalance exceeding the tolerable amount, the centrifuge halts rotations of the rotor before the device malfunctions.
One such sensor is an accelerometer that is mounted on the drive unit in the centrifuge for measuring accelerations to detect wobble (vibration or oscillation during rotation) in the rotor caused by an imbalance. The accelerometer is now being used for detecting vibrations caused by imbalances exceeding the tolerable amount and abnormal vibrations caused by operator error. (For example, some centrifuges generate self-excited vibration when the user does not firmly fix the rotor onto the rotating shaft.) However, the output obtained by an accelerometer is acceleration, calculated as follows.α=−Aω2 sin ωt (α: acceleration, A: amplitude, ω: angular velocity)
Hence, output for acceleration at low speeds (when ω is small) is low, making it difficult to establish a threshold for detecting vibrations.
Japanese patent application publication No. 2002-306989 (kokai) discloses a technique for overcoming this deficiency, wherein an amplification circuit is provided for amplifying signals outputted by the accelerometer, and the gain is modified according to the rotational speed.
By amplifying output at low speeds using the technique described above, detection near the resonance point (Nc) at low speeds is possible. This technique can also incorporate other techniques, such as varying the threshold for detecting imbalance based on the type of rotor. Hence, this technology has begun to be employed in a wide variety of centrifuges. With this method, a simple construction can be used to detect with great accuracy imbalances exceeding the allowable amount according to design specifications.